1. Field of the Invention
The present invention relates to a laminate having a porous organic semiconductor layer, and a chemical sensor comprising the same.
2. Description of the Related Art
An organic thin film transistor is a thin film transistor using an organic semiconductor, instead of inorganic (silicon) semiconductor, in its channel. There are no significant differences in overall structure between organic thin film transistors and silicon-based transistors. When a voltage is applied to the gate electrode, an electric field is generated in the semiconductor with an electric flow blocked by the insulating film, which leads to the semiconductor functioning as a field-effect transistor.
Turning to the operation principal of the device, applied voltage on the gate electrode controls the amount of charge carriers flowing between the source and the drain by forming a charge-free depletion layer or a charge-rich accumulation layer on the insulator/semiconductor interface. An on/off ratio, which is a current ratio taken when the device is operating relative to when the operation is stopped, is an important factor especially for applications of digital circuits in displays, such as computer monitors. Given an organic thin film transistor (OTFT) as a backplane, a display may have a bendable screen because OTFT allows for the use of a plastic plate as a screen substrate.
In spite of many years of extensive research into organic thin film transistors, the commercialization of organic thin film transistors has not yet been realized. Many companies that focused their competence on the development of OTFTs, such as ORFID, Motorola, Seiko Epson, PrintedSystems GmbH, PolymerVision, etc., have gone bankrupt, or have shifted to other fields. They seemed to have determined that OTFTs would not survive competition with LIPS (Low-temperature poly silicon) or oxide semiconductors (ZnO, CuO, Fe2O3, etc.) due to the low charge mobility and instability of organic semiconductors.
In recent years, however, the advent of high-performance small- or large-molecule materials for organic semiconductors has changed the situation and now prospects seem optimistic. Accordingly, synthesis techniques to maximize the performance of developed materials and device techniques for self assembly and performance are in demand. Particularly, deposition and solution processes for the fabrication of ultra thin films as thin as a single crystal are important material techniques to elicit maximum electrical performance from the materials, and are often introduced in the world's top journals.
Among the main barriers to the commercialization of organic thin film transistors are low crystallinity, poor quality such as unfavorable molecular arrangement, and relatively low mobility attributable to organic thin films. As a rule, the semiconductor-insulator interface has a great influence on the performance of the thin film transistor. Particularly for a bottom-gate transistor, surface properties of an insulator thin film play a critical role in controlling the growth and morphology of the semiconductor layer.
In addition to fundamentally having a switching function, such an organic thin film transistor may be utilized applied as a sensor device. On the whole, transistor-type sensors exhibit good sensitivity thanks to the application function that transistors fundamentally have. For transistor-type chemical sensors, a significant signal change cannot be brought about without the diffusion of analyte molecules to the transistor channels. However, because it is difficult for analyte molecules to pass through the semiconductor layer in conventional transistor-type chemical sensors of a bottom-gate structure, the sensors encounter the problem of low sensitivity and slow response rate.
In order to improve the sensitivity and response rate, crystal grains of semiconductor thin films are minimized to provide an intergrain path necessary for molecular diffusion whereby analyte molecules can readily pass through the organic semiconductor films. This strategy, however, reaches the impasses that the small crystal grain size of the organic semiconductor thin films and the intergrain pores do not guarantee sufficient electrical properties to the sensor function.